What Is a Hard Drive and How Does It Work?

By: Marshall Brain  | 
A hard disk inside a computer hard drive.
Hard disks store digital information in a relatively permanent form. See more computer hardware pictures.

Nearly every desktop computer and server in use today contains one or more hard-disk drives. Every mainframe and supercomputer is normally connected to hundreds of them, while even VCR-type devices and camcorders use hard disks instead of tape. So, what is a hard drive, and how does it operate these countless hard disks?

A hard disk drive is a data storage device that uses one or more rigid rapidly rotating disks coated with magnetic material to retrieve and store data. Hard disks do one thing well — they provide storage space for data and operating systems. They also give computers the ability to remember things when the power goes out. In this article, we'll take apart a hard drive so that you can see what's inside, and also discuss how they organize the gigabytes of information they hold in files!


The Basics of Traditional Hard Disk Drives

The invention of the hard disk drive in 1956 marked a revolutionary leap in data storage technology. Developed by IBM engineer Reynold B. Johnson and his team, the first hard disk drive was known as the IBM Model 350 Disk File, part of the IBM 305 RAMAC computer system. This groundbreaking device utilized magnetic disks for data storage, allowing for rapid access to large amounts of information compared to the sequential access of tape drives.

The original unit was as large as two refrigerators and stored a mere 5 megabytes of data on fifty 24-inch disks. Over the decades, HDDs have evolved dramatically in terms of capacity, size, and speed, significantly shaping the landscape of modern computing and data management.


Hard disks started as large disks up to 20 inches in diameter holding just a few megabytes. They were originally called "fixed disks" or "Winchesters" (a code name used for a popular IBM product). They later became known as "hard disks" to distinguish them from "floppy disks." Hard disks have a hard platter that holds the magnetic medium, as opposed to the flexible plastic film found in a tape and floppy disk.

At the simplest level, a hard disk is not that different from a cassette tape. Both hard disks and cassette tapes use the same magnetic recording techniques described in How Tape Recorders Work. Hard disks and cassette tapes also share the major benefits of magnetic storage — the magnetic medium can be easily erased and rewritten, and it will "remember" the magnetic flux patterns stored onto the medium for many years.

In the next section, we'll talk about the main differences between cassette tapes and hard disks.


Cassette Tape vs. Hard Disk

Let's look at the big differences between two types of storage devices — cassette tapes and hard disks:

  • The magnetic recording material on a cassette tape is coated onto a thin plastic strip. In a hard disk, the magnetic recording material is layered onto a high-precision aluminum or glass disk. The hard-disk platter is then polished to mirror-type smoothness.
  • With a tape, you have to fast-forward or reverse to get to any particular point on the tape. This can take several minutes with a long tape. On a hard disk, you can move to any point on the surface of the disk almost instantly.
  • In a cassette-tape deck, the read/write head touches the tape directly. In a hard disk, the read/write head "flies" over the disk, never actually touching it.
  • The tape in a cassette-tape deck moves over the head at about 2 inches (about 5.08 cm) per second. A hard-disk platter can spin underneath its head at speeds up to 3,000 inches per second (about 170 mph or 272 kph)!
  • The information on a hard disk is stored in extremely small magnetic domains compared to a cassette tape's. The size of these domains is made possible by the precision of the platter and the speed of the medium.

­Because of these differences, the storage capacity of a modern hard disk is pretty amazing. A hard disk can also access any of its information in a fraction of a second.


Storage Capacity and Performance

Typical desktop computers will have a hard disk with a capacity of between 10 and 40 gigabytes. Data is stored onto the disk in the form of files. A file is simply a named collection of bytes. The bytes might be the ASCII codes for the characters of a text file, or they could be the instructions of a software application for the computer to execute, or they could be the records of a data base, or they could be the pixel colors for a GIF image.

No matter what it contains, however, a file is simply a string of bytes. When a program running on the computer requests a file, the hard disk retrieves its bytes and sends them to the CPU one at a time.


There are two ways to measure the performance of a hard disk:

  • Data rate - The data rate is the number of bytes per second that the drive can deliver to the CPU. Rates between 5 and 40 megabytes per second are common.
  • Seek time - The seek time is the amount of time between when the CPU requests a file and when the first byte of the file is sent to the CPU. Times between 10 and 20 milliseconds are common.

The other important parameter is the capacity of the drive, which is the number of bytes it can hold.


Inside: Electronics Board

The backside of a Western Digital hard disk drive.
Warning: Don't look inside a hard disk unless you want to ensure it never works again!

The best way to understand how a hard disk works is to take a look inside. (Note that OPENING A HARD DISK RUINS IT, so this is not something to try at home unless you have a defunct drive.)

Here is a typical hard-disk drive:


­It is a sealed aluminum box with controller electronics attached to one side. The electronics control the read/write mechanism and the motor that spins the platters. The electronics also assemble the magnetic domains on the drive into bytes (reading) and turn bytes into magnetic domains (writing). The electronics are all contained on a small board that detaches from the rest of the drive:

Two internal halves of a hard disk drive, showing the board.
The inside of a hard disk drive.


Inside: Beneath the Board

Warning sticker on a hard disk stating "Warranty Void If Removed".
Hard disks won't work after they've been opened up.

Underneath the board are the connections for the motor that spins the platters, as well as a highly-filtered vent hole that lets internal and external air pressures equalize:

Removing the cover from the drive reveals an extremely simple but very precise interior:


Inside of a hard disk drive.
The inside of a hard disk.

In this picture you can see:

  • The disk platters - These typically spin at 3,600 or 7,200 rpm when the drive is operating. These platters are manufactured to amazing tolerances and are mirror-smooth (as you can see in this interesting self-portrait of the author... no easy way to avoid that!).
  • The arm - This holds the read/write heads and is controlled by the mechanism in the upper-left corner. The arm is able to move the heads from the hub to the edge of the drive. The arm and its movement mechanism are extremely light and fast. The arm on a typical hard-disk drive can move from hub to edge and back up to 50 times per second — it is an amazing thing to watch!


Inside: Disk Platters and Heads

Disk platters on a computer hard disk drive.
A close up of an exposed hard disk drive.

In order to increase the amount of information the drive can store, most hard disks have multiple platters. This drive has three disk platters and six read/write heads:

Three disk platters on a hard disk drive.
Here are three disk platters on a hard disk drive.

The mechanism that moves the arms on a hard disk has to be incredibly fast and precise. It can be constructed using a high-speed linear motor.


The arm and motor on an exposed hard disk drive.
The arm and motor on an exposed hard disk drive.

Many drives use a "voice coil" approach — the same technique used to move the cone of a speaker on your stereo is used to move the arm.


Storing Data

Illustration of spinning platters on a hard disk drive.
Illustration showing the sectors and tracks of a hard disk drive.

Data is stored on the surface of a platter in sectors and tracks. Tracks are concentric circles, and sectors are pie-shaped wedges on a track, like this:

A typical track is shown in yellow; a typical sector is shown in blue. A sector contains a fixed number of bytes -- for example, 256 or 512. Either at the drive or the operating system level, sectors are often grouped together into clusters.


The process of low-level formatting a drive establishes the tracks and sectors on the platter. The starting and ending points of each sector are written onto the platter. This process prepares the drive to hold blocks of bytes. High-level formatting then writes the file-storage structures, like the file-allocation table, into the sectors. This process prepares the drive to hold files.

For more information on hard disks and related topics, check out the links on the next page.

This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.


Frequently Answered Questions

What is HDD vs SSD?
HDD is a hard disk drive, which stores data on a spinning disk. SSD is a solid state drive, which stores data on a chip. SSD is faster and more durable than HDD.
What is hard disk example?
A computer's internal hard drive is an excellent example of a hard disk. This disk is typically mounted inside the computer case on top of the power supply unit. The hard disk contains spinning disks (called platters) that store data on their surfaces.
What is a hard disk used for?
A hard disk is a storage device used for storing and retrieving digital information.